1. Field of the Invention
The present invention generally relates to a current control device for regulating current flow via compression and expansion of a composite.
2. Related Arts
Mechanical circuit breakers are best described as a switch wherein a contact alters the electrical impedance between a source and a load. Mechanical breakers are typically composed of a snap-action bimetal-contact assembly, a mechanical latch/spring assembly, or an expansion wire. Such devices are neither gap-less nor shock resistant, therefore prone to chatter and subject to arcing. Chatter and arcing pose substantial problems in many high-voltage applications.
Variably conductive composites are applicable to current control devices. Compositions include positive temperature coefficient resistive (PTCR), polymer current limiter (PCL), and piezoresistive formulations.
PTCR composites are composed of a conductive filler within a polymer matrix and an optional nonconductive filler. Chandler et al., U.S. Pat. No. 5,378,407, describes and claims a PTCR composite having a crystalline polymer matrix, a nickel conductive filler, and a dehydrated metal-oxide nonconductive filler. Sadhir et al., U.S. Pat. No. 5,968,419, describes and claims a PTCR composite having an amorphous polymer matrix, a thermoplastic nonconductive filler, and a conductive filler. During a fault, the composite heats thereby increasing volumetrically until there is sufficient separation between particles composing the conductive filler to interrupt current flow. Thereafter, the composite cools and shrinks restoring conduction. This self-restoring feature limits PTCR compositions to temporary interrupt devices.
PCL composites, like PTCR compositions, are a mixture of a conductive filler and a polymer. However, PCL composites are conductive when compressed and interrupt current flow by polymer decomposition. For example, Duggal et al., U.S. Pat. No. 5,614,881, describes a composite having a pyrolytic-polymer matrix and an electrically conductive filler. During a fault, temperature within the composite increases causing limited decomposition and evolution of gaseous products. Current flow is interrupted when separation occurs between at least one electrode and conductive polymer. Gap dependent interrupt promotes arcing and arc related transients. Furthermore, static compression of the composites increases time-to-interrupt by damping gap formation. Neither PTCR nor PCL applications provide for the dynamically-tunable compression of a composite in response to electrical load conditions.
Piezoresistive composites, also referred to as pressure conduction composites, exhibit pressure-sensitive resistivity rather than temperature or decomposition dependence. Harden et al., U.S. Pat. No. 4,028,276, describes piezoresistive composites composed of an electrically conductive filler within a polymer matrix with an optional additive. Conductive particles comprising the filler are dispersed and separated within the matrix, as shown in FIGS. 1A and 1C. Consequently, piezoresistive composites are inherently resistive becoming less resistive and more conductive when compressed. Compression reduces the distance between conductive particles thereby forming a conductive pathway, as shown in FIGS. 1B and 1D. The composite returns to its resistive state after compressive forces are removed. However, piezoresistive compositions resist compression.
Pressure-based interrupt facilitates a more rapid regulation of current flow as compared to PTCR and PCL systems. Temperature dependent interrupt is slowed by the poor thermal conduction properties of the polymer matrix. Decomposition dependent interrupt is a two-step process requiring both gas evolution and physical separation between electrode and composite. Furthermore, decomposition limits the life cycle of a composition.
Active materials, including but not limited to piezoelectric, piezoceramic, electrostrictive, and magnetostrictive, are ideally suited for the controlled compression of piezoresistive composites thereby achieving rapid and/or precise changes to resistivity. Active materials facilitate rapid movement by mechanically distorting or resonating when energized. High-bandwidth active materials are both sufficiently robust to exert a large mechanical force and sufficiently precise to controllably adjust force magnitude.
As a result, an object of the present invention is to provide a current control device tunably and rapidly compressing a pressure-dependent conductive composite. A further object of the present invention is to provide a device that eliminates arcing thereby facilitating a complete current interrupt. It is an additional object of the present invention to provide a device that quenches transient spikes associated with shut off.